Steps to replicate:

1. Navigate to a course page.
2. Turn on Edit Mode.
3. Click the Add a block button.
4. Click the "Calendar" block.
5. Right-click the mouse on the "Add a block" button.
6. You will be redirected to another page.
7. Click the "Calendar."
8. You will be redirected to the previous page.
9. You can see the calendar block has been duplicated.

Actual:
You can see the calendar block has been duplicated.

Expected:
Calendar block or any blocks cannot be duplicated.